<!DOCTYPE html>
<html>
<head>
	<title></title>
	<style type="text/css">
	.o1{
		/*background: red;
		opacity: .5;*/
		background: rgba(255,0,0,.5);
	}
	</style>
</head>
<body>

<div class='o1'>
	这是o1111
	<div class='o2'>
		这是o2222
	</div>
</div>

</body>
</html>